Creepy movment with IK
Whenever your bones kind of go off and do their own thing, 9/10 times it is because there is a cyclical dependency somewhere in your armature hierarchy.  The easiest way to diagnose this is to turn on the system console (Windows only, as far as I can tell).



From there, it will usually just tell you.  



Fixing it is a matter of reorganizing your bone dependencies to not be self-referential.  What that means precisely depends on the model, but remember that an IK control bone cannot be parented to a bone it controls (i.e. a IK control foot cannot be parented to the shin it controls through IK).